1. Field of the Invention
This invention relates to a method of forming an electrostatic image, and more particularly to a method of forming an electrostatic latent or visible image in which a control electrode assembly comprising a pair of parallel spaced perforated electrodes interposing an insulating layer therebetween is used to modulate a corona ion current or a toner particle current to form an electrostatic image on an electrostatic recording medium.
2. Description of the Prior Art
In an electrostatographic method as disclosed in preprints for "5th National Conference of Society of Image Electronics" (1977) entitled "Investigation of Facsimile Receiver Using Ion Current Electrostatic Recording" and Japanese Patent Publication No. 20094/1975, an electric voltage is applied to a control electrode assembly comprising an insulating layer and a pair of parallel perforated conductive electrodes interposing the layer to establish an electric field within the perforations thereof and the electric field serves to modulate a corona ion current passing through the perforations to form an electrostatic latent image on an electrostatic recording medium.
The above conventional method of forming an electrostatic latent image using an ion current will hereinbelow be described in more detail referring to FIGS. 1 and 2. In FIG. 1, a corona discharger 1 having a wire anode and a cylindrical cathode provides corona discharge between the anode and the cathode. The corona ions generated by the corona discharger 1 are attracted to a backside electrode 2 travelling through a hole 9 in a control electrode assembly 6 disposed between the corona discharger 1 and the backside electrode 2. High voltages are applied to the anode of the corona discharger 1 and the backside electrode 2 from high voltage sources 3 and 4, respectively. An electrostatic recording paper 5 is supported on the backside electrode 2.
The control electrode assembly 6 comprises an insulating plate 7 and a pair of copper films 8a and 8b attached to opposite surfaces thereof. The copper films 8a and 8b function as a pair of conductive electrodes, respectively. The control electrode assembly 6 is provided with at least one through hole 9 having a small diameter. An electric field is established within the through hole 9 when an electric voltage is applied between the conductive electrodes 8a and 8b. As will be described in more detail hereinbelow, a corona ion current passing through the hole 9 can be controlled by controlling the direction of the electric field established within the hole 9. Thus the corona ion current passing through the control electrode assembly 6 is modulated by controlling the direction of the electric field applied to the electrodes 8a and 8b to form an electrostatic latent image in the form of a dot pattern on the electrostatic recording paper 5. When the control electrode assembly 6 is provided with a number of such holes, the upper conductive electrode 8a is devided into portions each surrounding one of such holes.
Upon formation of the electrostatic latent image, a high voltage of several KV is applied to the backside electrode 2 and a voltage of several tens of volts (assuming that the hole 9 has a diameter of several tens of microns) is applied between the conductive electrodes 8a and 8b of the control electrode assembly 6 by a signal source 10. Then an electric field E.sub.F is established within the hole 9 and an electric field E.sub.P is established between the corona discharger 1 and the backside electrode 2 as shown in FIG. 2. The corona ions generated by the discharger 1 pass through the hole 9 depending on the vector sum of the vectors of the fields E.sub.F and E.sub.P. Thus, it is possible to control the ion current passing through the hole 9 by controlling the direction of the vector of the electric field E.sub.F in the hole 9.
The above described electrostatic recording method using the control electrode assembly 6 can be conveniently used with an electrostatic recording medium which can be used repeatedly.
Generally, in a conventional electrostatic recording system, a discharge electrode is brought into contact with an electrostatic recording medium or is spaced therefrom by a distance of several tens of microns. Such close positioning of the discharge electrode does not bring about any problem in case that the recording medium is not repeatedly used as in facsimile. However, in case that the recording medium is repeatedly used, a serious problem arises due to the extremely small space between the discharge electrode and the recording medium. While the recording medium is repeatedly subjected to processes of latent image formation, development, transfer of image and cleaning, the small space between the electrostatic recording medium and the discharge electrode will be clogged with the residual toner particles and/or dusts. This will lower the quality of the obtained image and in the worst case it would prevent formation of an electrostatic latent image. The cleaning, therefore, should be conducted perfectly. However, it is practically impossible to perfectly clean the surface of the recording medium, and there would be required frequent maintenance of the system.
The electrostatic recording method using the control electrode assembly is advantageous in that the control electrode assembly can be spaced from the recording medium by a distance of one to several millimeters and accordingly the perfectness of the cleaning would not be required.
However, this method has a disadvantage that the voltage which should be applied to the control electrode assembly must be increased when the diameter of the hole 9 is increased. In order to obtain the increased voltage, a complicated switching circuit would be required.
Generally, the effectiveness of the electric field preventing the corona current from passing through the hole 9 is reduced as the thickness of the control electrode assembly is reduced for a given diameter of the hole 9. The effectiveness of the electric field can be increased by increasing the voltage applied to the control electrode assembly. However, an increased voltage would generate electric sparks within the control electrode assembly. The electric field should normally be of several to several tens of KV/cm. Thus a high voltage as high as from several hundred V to several KV would be required when the diameter of the hole 9 is about 1 mm. In order to conduct switching of such a high voltage, an extremely complicated costly switching circuit is required.